1. Scope of the Invention
The present application relates to colloidal suspensions of zeolite synthesized in clear homogeneous aluminiumsilicate solutions. The particles are present as discrete particles, with an average particle size of less than 250 nanometers and preferably, less than 200 nanometers with a particle size distribution expressed as the geometric standard deviation of less than 1.30 and preferably less than 1.20. The colloidal zeolite sols possess characteristics typical of colloidal suspensions such as for example, a very low rate of sedimentation and Tyndall light scattering. The zeolite sols can be prepared so as to contain essentially no amorphous material. Such zeolite sols can be dried to a powder suitable for use in numerous fields of application such as the active component in fluid catalytic cracking catalysts. Equilibrium and demetallized fluid catalytic cracking catalysts may be upgraded by means of impregnation of such catalysts using a suitable colloidal zeolite sol as for example, a colloidal suspension of zeolite Y. The particles of this invention can be deposited on suitable substrates in the production of inorganic films and membranes which possess molecular sieve characteristics. Such membranes would possess a bi-modal pore structure where the size of the larger pores is a function of the zeolite particle size while the size of the smaller pores is determined by the type of zeolite employed. The use of the sols of this invention allows for the production of films and membranes with controlled acid-base and ion-exchange properties.
2. A Description of the Prior Art
Crystalline aluminiumsilicates or zeolites are normally synthesized from active hydrated aluminiumsilicate gels in an alkaline enviroment. The synthesis of crystalline aluminiumsilicates is achieved by mixing a silica- and an alkaline aluminate solution, thereby obtaining an aluminiumsilicate gel.
Suitable silica sources are for example hydrated silicates, precipitated silica powders and colloidal silica sols. The alumina source may be aluminium alkoxides, aluminium salts, aluminium oxides, aluminium hydroxide or metal aluminates. The necessary alkalinity is supplied by additions of alkali hydroxides, alkaline earth hydroxides or organic bases or combinations thereof.
U.S. Pat. No. 3,130,007 describes the preparation of silica-alumina synthesis gels for the synthesis of zeolite Y wherein the alkali is added as sodium hydroxide. The method entails ageing of the gel obtained followed by hydrothermal treatment whereupon zeolite Y crystallizes with a particle size of between 1 and 7 micrometers.
U.S. Pat. Nos. 3,639,099 and 4,164,551 describe a method whereby zeolite Na-Y is synthesized without ageing of the synthesis mixture. A seed mixture, which is obtained as a gel with a relatively high alkalinity, is added to a synthesis mixture which upon hydrothermal treatment yields zeolite Na-Y. The product, in the form of a sediment, is separated from the mother liquid and dried to a powder after a washing sequence.
U.S. Pat. No. 3,411,874 describes the synthesis of zeolite ZSM-2 using a lithium aluminiumsilicate gel. The product is described as containing a mixture of 50% zeolite ZSM-2 with an average particle size of 0.5 micrometer and 50% unreacted amorphous lithium aluminiumsilicate glass.
The colloidal zeolite suspensions referred to in this invention are obtained from clear homogeneous aluminiumsilicate synthesis mixtures. Such clear homogeneous aluminiumsilicate synthesis mixtures are however not the sole criteria to be fullfilled in order to synthesize discrete colloidal zeolite crystals. TPA-silicalite-1 can be crystallized in clear homogeneous solutions as described by Cundy, Lowe and Sinclair, J. Cryst. Growth, 100, (1990), 189. The crystals thus obtained were separated from the mother liquid by sedimentation and washed free from excess alkali by filtering through a 0.2 .mu.m filter membrane. Separation according to this technique was possible due to the fact that the product consisted of silicalite particles with an average particle size of 0.95 .mu.m. The particle size distribution could be determined by means of optical microscopy.
A further example of a zeolite that crystallizes in clear homogeneous synthesis solutions is the synthesis of zeolite Y as described by Kashara, Itabashi and Igawa, `New Developments in Zeolite Science and Technology`, (Ed. Murakami et.al.), Elsevier, Proc. of the 7th Int. Conf. on Zeolites, (1986 ), 185. Aluminiumsilicate seeds or nuclei are formed in a clear solution and grow by means of aggregation. As a result of the particle size, the growing crystals sediment thus allowing separation of the crystalline phase by means of conventional filtration methods well known in the art.
Ueda et.al., Am. Mineral, 64, (1979 ), 172, show that analcime crystallizes in clear homogeneous solutions. The product consists of analcime particles with an average particle size of between 15 and 25 0.mu.m.
Wenqin, et.al., `New Developments in Zeolite Science and Technology`, (Ed. Murakami et.al.), Elsevier, Proc. of the 7th Int. Conf. on Zeolites, (1986), 177, describe the synthesis of zeolite A from clear homogeneous solutions. The product thus obtained could be separated from the mother liqour by means of conventional filtration methods which implies that the average particle size is markedly larger than the particle sizes referred to in this invention.
Zeolite crystals synthesized with typical synthesis conditions as those described above have an average particle size of between 1 and 5 .mu.m. The particle size distribution is as a rule broad. Characterization of such particles with respect to particle size is accomplished with methods such as optical or electron microscopy, sieve analysis and light scattering methods suitable for particles in this size range. Determining the diffusion coefficient by means of light scattering in order to calculate the particle size according to equation 1 is not a suitable method since these particles have a rate of sedimentation far greater than the Brownian motion. EQU D=kT/3.eta..pi.d Equation 1
where
D=diffusion coefficient, m.sup.2 /s PA1 k=Boltzmanns constant=1.38 10.sup.-23 J/K PA1 T=absolute temperature, K PA1 .eta.=liquid phase viscosity, Ns/m.sup.2 PA1 d=particle diameter, m. PA1 L=average particle size, nanometer PA1 K=form factor=0.893 PA1 .lambda.=CuK.alpha. radiation wavelength=0.15405 nm PA1 2.THETA.=diffraction angle PA1 B=peak broadening due to sample, radians PA1 .beta.=instrument broadening, radians.
The characteristics of different particulate systems may be compared if the particle size distributions are expressed as a lognormal particle size distribution. The systems are defined by an average particle size and a geometric standard deviation, GSD. The GSD in a colloidal suspension of amorphous aluminiumsilicate particles prepared according to U.S. Pat. Nos. 4,257,874 and 4,272,409 is 1.37. The colloidal suspensions described in these patents are said to contain discrete amorphous aluminiumsilicate particles with a narrow particle size distribution. With a narrow particle size distribution referred to in this invention, a GSD of less than 1.30 or preferably less than 1.20 is intended.
Zeolite particles in conventional zeolite syntheses can exist as agglomerates of smaller particles. Such particles do not display the typical properties of colloidal suspensions as described above and, furthermore, which are typical for the colloidal zeolite referred to in this invention.
The product in conventional zeolite syntheses can contain a crystalline zeolite phase as well as an amorphous, unreacted aluminiumsilicate if crystallization has not proceeded to completion or if one or more of the components exist as excess reactant. It is not possible or very difficult to separate the crystalline component from the largely insoluble amorphous component with separation techniques known in the art. The resulting consequence of this fact is that it is not practical to interrupt a conventional zeolite synthesis at some intermediate stage during the course of crystallization in order to obtain crystals with a colloidal size or with a particle size less than that which would have resulted had the crystallization proceeded to completion.
Zeolite crystals synthesized according to the conventional methods described above and known in the art, display a high rate of sedimentation which is a direct consequence of their particle size. The particulate material obtained from such conventional synthesis methods can be separated from the mother liquor by means of conventional methods of separation such as filtration. It is the presence of large particles that allows such methods to be employed in contrast to the more sophisticated separation methods such as high speed centrifugation or ultrafiltration that have to be employed to separate the colloidal material referred to in this invention.
General methods known and employed to characterize and describe colloidal suspensions containing discrete particles in the colloidal size range, 10 to 10 000 .ANG., for example the particles diffusion coefficient and the suspensions critical coagulation concentration, cannot be applied to the corresponding zeolites synthesized according to the conventional methods known in the art and which yield zeolite particle sizes in excess of 1 .mu.m. The above practice is on the other hand applicable to the colloidal suspensions of crystalline aluminiumsilicates described in the present invention.